The invention relates to an electrodynamic transducer for non-destructive material testing by ultra-sonic means with an electro-magnet having a core surrounded by a magnetic winding in a fixed position in the transducer, and at least one internal pole shoe extending from the core and one corresponding external pole shoe, the internal pole having excitation and receiver coils arranged on the end section thereof facing the electrically conductive workpiece, which is to be tested.
In order to be able to test sheet metal, tank floors and other electrically conductive flat objects, it is desirable to use testing methods in which the necessary number of test heads is small, that is to say, providing the largest possible testing track width. To satisfy these requirements eddy-current transmitters or ultra-sonic test heads with water coupling can be used, whereby the probes are arranged to oscillate, or to rotate on a disc (Material Test 16 (1974) No. 9, September, pages 281-282). However, the disadvantage of eddy-current probes is that only defects close to the surface can be detected. Against this eddy-current probes cannot carry out any dimension measurements. Moreover, eddy-current probes are insensitive to the frequently occurring doubling effects. When using piezo test heads it is necessary to couple via a water path, whereby it is difficult and technically extravagant to achieve a rotating system.
Moreover, with oscillating systems, due to the oscillation frequency (1-2 Hz), it is either impossible or only possible at great expense to achieve a high testing speed by multiplying the number of heads.
If one uses the electrodynamic transducer described above (e.g., as described in DE-OS No. 31 23 935 or EP-A2-No. 0045412), one can determine flaws and thickness without needing to couple via a water path. In these transducers the internal pole shoe is always arranged in rotational symmetry with the center axis of the core of the electromagnet, whereby excitation and receiver coils can be arranged on the free end of the internal pole shoe, or to the side thereof. However, the high weight of such heads gives rise to large centrifugal forces when rotating, or high acceleration when oscillating.